Field of the Invention
The present invention relates to an anti-reflection coat preventing the reflection of incident light by utilizing optical interference effect and an optical device provided with the anti-reflection coat.
Background Art
On the surface of the substrate of an optical device such as a lens or a prism constituting an optical apparatus, an anti-reflection coat is provided to enhance the optical transmittance. The anti-reflection coat reduces the reflection of incident light mainly utilizing optical interference effect. In an anti-reflection coat composed of a single layer, part of the incident light reflects at both the surface of the anti-reflection coat and the interface between the anti-reflection coat and the substrate. When the optical film thickness of the anti-reflection coat is quarter of the incident light wavelength (Lambda), the phase of the interface-reflected light is reversed in relation to the phase of the surface-reflected light, and due to the optical interference effect, the surface-reflected light and the interface-reflected light cancel each other. So, when the incident medium is air, the refractive index of the substrate is denoted by n(sub) and the refractive index of the anti-reflection coat is denoted by the square root of n(sub), reflectance of the incident light having the wavelength of λ is made to be 0%. However, such an optical design can assure the low reflectance only in a narrow band (in the vicinity of the reference wavelength).
So, formation of a multilayer film composed of a plurality of layers having different refractive indexes is required to provide an anti-reflection coat effective in wide-band wavelength light. As a low refractive index layer provided at the interface between the multilayer film and the air, a vapor deposition film using an inorganic material is generally used; specifically, magnesium fluoride film having a refractive index of about 1.38 or silica film having a refractive index of about 1.49 are used. The performance of an anti-reflection coat is largely affected by the refractive index of the low refractive index layer provided at the interface with the air; i.e. the lower the refractive index, the higher the antireflection performance. However, as the materials which can be used in the vapor deposition method for film formation are limited, it has been difficult to achieve a further lower refractive index in a film formed by using a vapor deposition method. Then, materials for wet film formation method which contains air in the films have been developed in recent years, and low refractive index layers having a refractive index of 1.15 to 1.35 have been achieved by wet film formation method.
Further, most of conventional optical apparatuses have been used for light beams incident in specific narrow incident angle ranges. So, the optical design employed for an anti-reflection coats was excellent in antireflection effect in a specific incident angle range. However, depending on the miniaturization and high performance of lenses, lenses having wide aperture and large curvatures have been used in recent years. When a lens having a large curvature is used, the light beam incident angle range in the peripheral portion of the lens enlarges. Consequently, anti-reflection coat excellent in antireflection effect for light beams in the whole visible light region which incidents in a wider incident angle range is required.
From such a viewpoint described above, for example, Japanese Patent No. 4378972 (hereinafter; Patent Document 1) proposes an anti-reflection coat composed of a low refractive index layer having refractive index of 1.20 to 1.50 formed by a wet film formation method using a hollow fine particle. In Patent Document 1, a low refractive index is achieved by using a hollow fine particle as the film formation material and introduces voids in the layer. Patent Document 1 also states that the durability of the anti-reflection coat is enhanced by adhering hollow fine particles each other by a first binder followed by filling the voids between the hollow fine particles in a filling ratio of 40% or more by a second binder.
Next, Japanese Patent Laid-Open No. 2006-215542 (hereinafter; Patent Document 2) discloses an anti-reflection coat having a double layer structure including a dense layer and a porous silica aerogel layer sequentially provided in this order on a single layer substrate. In the anti-reflection coat disclosed in Patent Document 2, the surface-reflected light is canceled by utilizing the interface-reflected light generates at the interface between the individual layers by changing the refractive index in a smooth step-like manner from the substrate toward the medium in the optical film thickness direction, and even when the wavelength of the incident light is in a wide band and the incident angle of the incident light is over a wide range, excellent antireflection effect is achieved.
By the way, the low refractive index layer disclosed in Patent Document 1 and the porous silica aerogel layer disclosed in Patent Document 2 are both formed by a wet film formation method. However, the wet film formation method is hard to accurately form an anti-reflection coat on the surface of a lens, a curving surface. In particular, it is extremely difficult to accurately form an extremely thin anti-reflection coat on the surface of a lens having a large curvature. When surface defect such as micro-protrusions due to foreign matter or micro-recesses due to flaws on the surface of a lens exist, the unevenness in the film thickness generates over wide area from such defect sites. Consequently, the anti-reflection coat may have large quality deviation, poor antireflection performance or poor appearance.
Next, the anti-reflection coat disclosed in Patent Document 1 adopts a method for improving the durability by filling the voids between the hollow fine particles with the second binder. So, the higher the void fraction in the layer, the lower the refractive index but the more poor in the durability at the same time. On the other hand, the higher the filling ratio by the second binder, i.e. the lower the void fraction in the layer due to the second binder, durability might be enhanced but refractive index increases. As described above, as the refractive index and the durability are in a trade-off relation; it is difficult to achieve excellent antireflection performance because refractive index of the layer cannot be largely decreased to obtain a practically sufficient durability.
Moreover, in the anti-reflection coat disclosed in Patent Document 1, adhesion of the low refractive index layer to the substrate is not sufficiently investigated, and adhesion of the low refractive index layer to the substrate may be low. Patent Document 1 further discloses a structure in which the low refractive index layer is provided via a hard coat layer to enhance adhesion of the low refractive index layer to the substrate. However, as the hard coat layer is formed by a wet film formation method also, the hard coat layer uniform in thickness may hardly be formed on a lens having a large curvature. Further, in the anti-reflection coat disclosed in Patent Document 1, the hard coat layer does not perform as an optical interference layer. So, the anti-reflection coat disclosed in Patent Document 1 is equal to a single-layered optical interference layer, and as described above, when the wavelength of the incident light deviates from the design wavelength, or when the incident angle range of the incident light extends over a wide range, a problem such that no sufficient antireflection performance is achieved in the anti-reflection coat may arise.
Next, trade-off relation between the refractive index and the durability lies in the anti-reflection coat disclosed in Patent Document 2 also. That is, when a porous silica aerogel layer will be formed on the surface of a SiO2 layer to have a refractive index of 1.15 as a dense layer, no practical durability is obtained due to the original property of silica aerogel. Further, it should be noted that silica aerogel changes its structure when moisture is adsorbed. Then, silica aerogel may be subjected to a hydrophobization treatment with a fluoride for preventing the adsorption of moisture, but the treatment increases refractive index of the porous silica aerogel layer. So, it has been reported that, for example, when silica aerogel particles which adhere each other by using a binder in the formation of the porous silica aerogel layer, refractive index should be about 1.25 to achieve practical durability (see, for example, “Proceedings of 35th Optical Symposium” under the auspices of the Optical Society of Japan, an affiliate of the Japan Society of Applied Physics, July 2010, pp. 67-70). Further, it has also been reported that when the anti-reflection coat having a refractive index about 1.25 is subjected to a high-temperature high-humidity test (60° C., 90% RH), reflectance increases by about 0.2%.